The present invention relates to the field of semiconductor processing; and more specifically to the field of conditioning methods and apparatuses for polishing pads used in the planarization of thin films formed on a semiconductor substrate.
Current integrated circuits (ICs) rely upon an elaborate system of metallization interconnects to couple various devices fabricated in and on the semiconductor substrate. These metallization interconnects are commonly formed, for example, by depositing aluminum or some other metal and (i.e., a first metal layer) then patterning the deposited metals to form interconnection paths along the surface of the silicon substrate. A dielectric or insulated layer, such as a chemical vapor deposition layer of (CVD) silicon dioxide, is then deposited over the first metal layer. The dielectric layer conformably covers the patterned first metal layer such that the upper surface of the dielectric layer is characterized by a series of non-planar xe2x80x9cstepsxe2x80x9d which correspond in height and width to the underlying patterned first metal layer. Openings, or vias, are then etched through the dielectric layer and a second metal layer is deposited on the dielectric layer so as to fill the via openings and complete an electrical connection (i.e., the metallization interconnects) between selected portions of the second metal layer and the first metal layer.
Step height variations in the upper surface of the interlayer dielectric are undesirable. For example, steps create a non-planar dielectric surface, which interferes with the optical resolution of subsequent photolithography processing steps and makes it extremely difficult to print high-resolution lines. In other words, if the upper surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar and the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. Therefore, various techniques have been developed to planarize the upper surface of the dielectric layer.
Chemical mechanical polishing is one accepted method of planarization. A conventional approach employs an abrasive polishing to remove the protruding steps along the upper surface of the dielectric. According to this method, a silicon substrate or wafer is forced faced down by quill on a table covered with flat pad coated with an abrasive material (slurry). Both wafer and table are rotated relative to each other under pressure to remove the protruding steps. The abrasive polishing process continues until the upper surface of the dielectric layer is planarized. However, the surface of the polishing pads used for wafer planarization quickly degrade or xe2x80x9cglazexe2x80x9d and are unable to consistently provide a desired polishing rate and uniformity. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against the polishing pad. The peaks of the polishing pad are pressed down and the pits of the polishing pad are filled making the polishing pad surface smooth and less abrasive. Therefore, to combat xe2x80x9cglazingxe2x80x9d pad conditioning is performed to restore the abrasiveness of the polishing pad.
One approach to pad conditioning utilizes a diamond-impregnated disk. In this approach, the disk is pressed against the polishing pad and rotated. These diamond-impregnated disks have about 100,000 diamonds having a substantially normal statistical distribution of absolute diamond heights of diamond tips to a common reference surface, which can conveniently be defined as a plane parallel to the disk surface which intersects the lowest point on the metal substrate surface, but may be defined relative to any other convenient reference point(s).
However, diamond-impregnated disks represent a significant source of CMP process variability as measured by initial removal rates, removal rate drift, and within wafer non-uniformity drift. This process variability stems from variability in the absolute diamond heights in the xe2x80x9cworking tailxe2x80x9d portion of the diamond disk shown in FIG. 1(a) and depends on factors such as disk front-side flatness, diamond size, and diamond shape. The xe2x80x9cworking tailxe2x80x9d includes diamonds with an absolute diamond height within a range of about 0.0005 inches and accounts for roughly 3% of the total diamond population. Only diamonds in this xe2x80x9cworking tailxe2x80x9d actually see or contact the polishing pad (i.e., the absolute diamond height window). In other words, within the xe2x80x9cworking tailxe2x80x9d, the diamond with the largest absolute height sits approximately 0.0005 inches higher than the diamond with the lowest absolute diamond height that still sees or contacts the pad. The vast majority of diamonds, approximately 97%, do not contact the pad and do not contribute to pad conditioning.
Additionally, as the small percentage of diamonds having the highest absolute diamond height wear, other diamonds do not become available. The wear mechanism is a rounding of the diamond cutting edge. After rounding occurs, the diamond rides on the pad surface, but does not xe2x80x9ccutxe2x80x9d. Given this wear mechanism and the substantially narrow absolute diamond height distribution of conventional diamond-impregnated disks, the disk cut rate decreases with disk use because the bulk diamond does not wear fast enough to introduce new cutting edges associated with diamonds disposed at slightly lower absolute diamond heights. Further, since the diamonds in the diamond-impregnated disk are not designed to break away to expose new diamonds as many other types of diamond polishing or cutting tools, the disk moves to a state where it is mostly riding on rounded diamonds.
Thus, diamond-impregnated disks suffer not only from drawbacks such as non-uniform polishing due to insufficient surface flatness or improper alignment, but exhibit decreased performance and service life due to the statistically normal distribution of absolute diamond heights and inability of all of the diamonds to contribute to the pad conditioning.
In another approach to pad conditioning, grooves are formed in a polishing pad by rotating a conditioning block having a number of discrete point contacts and moving it between an outer radius and an inner radius of the polishing pad corresponding to a wafer polishing area. These discrete point contacts, such as diamond tipped threaded shanks, extend from the bottom surface of the conditioning block and the position of the tips may be adjusted by a corresponding clockwise or counterclockwise rotation of the threaded shanks. However, it is difficult to obtain an even loading condition with discrete points mounted on a rigid conditioning blocks or disk due to uneven loadings on the discrete points caused by slight non-planarity of the tips of the discrete points (e.g., the diamond tips). This non-planarity has a deleterious effect on the conditioning uniformity, especially when the conditioning block has only a relatively small number of discrete polishing points, and is further aggravated by axial movement of the shanks caused by thermal or mechanical stresses.
Accordingly, a need exists for a pad conditioning apparatus capable of maintaining a plurality of conditioning elements at a substantially equal distribution of absolute diamond heights. Further, a need exists for a conditioning apparatus providing a substantially equal pressure distribution on the conditioning elements.
In one aspect, the invention features a pad conditioning assembly comprising a conditioner head with an end effector that is movable into contact with a polishing pad, a plurality of downwardly-projecting movable conditioning elements disposed at a bottom of the end effector; and a compliant backing member disposed above and adjacent the conditioning elements. Forces applied by the compliant backing member are transferred to the movable conditioning elements to move the conditioning elements. This aspect may also include, for example, a pressurization circuit for applying a pressure from a pressure source to the compliant backing member to flex the compliant backing member against the movable conditioning elements. The end effector may include a plurality of through holes formed in a bottom portion thereof and may include a chamber formed in an interior portion thereof adjacent the bottom portion so that at least one compliant backing member is disposed within the chamber and adjacent the bottom portion of the end effector. The plurality of movable conditioning elements may also comprise a flanged base portion, a shaft portion, and a tip portion arranged to be movable within the through holes.
In another aspect, the invention features a pad conditioning assembly comprising a conditioner head with an end effector that is movable into contact with a polishing pad, a plurality of conditioning elements, and a compliant backing member holding the conditioning elements on the end effector. This aspect may include a chamber formed within the end effector; and a pressurization circuit for supplying a pressure from a pressure source to the chamber to pressurize the chamber to a predetermined pressure, wherein the compliant backing member is disposed to cover an open portion of the chamber. Still further, the conditioning elements of this aspect may include a proximal end secured to the compliant backing member and a distal end terminating in a point, wherein pressurization of the chamber flexes the compliant backing member to provide a substantially uniform force distribution among conditioning elements engageable with a pad to be conditioned to regulate a penetration depth of the conditioning element points into the pad.
In still another aspect, the invention features a pad conditioning assembly comprising a conditioner head with an end effector that is movable into contact with a polishing pad, a plurality of downwardly-projecting pneumatically movable conditioning elements retained by the end effector, and a pressurization circuit for selectively changing a pressure within the end effector, wherein pressure changes in the end effector move the conditioning elements realtive to the end effector. The pressurization circuit may include a pressure source, a controller for controlling the chamber pressure, and a conduit between the pressure source and the end effector to provide a pressure from the pressure source to the end effector. The downwardly-projecting pneumatically movable conditioning elements may be positioned to have a substantially equal absolute diamond height.
In another aspect, the invention features a pad conditioning assembly comprising a conditioner head with an end effector movable into contact with a polishing pad and a plurality of downwardly-projecting movable conditioning elements retained by the end effector, wherein the plurality of conditioning elements are independently movable relative to the end effector during processing.
In yet another aspect, the invention features a method for conditioning a polishing surface of a polishing pad, comprising bringing a plurality of vertically movable conditioning elements having tip portions protruding from a bottom portion of the an effector and retained by the end effector into contact with the polishing pad; causing relative motion between the polishing pad and the conditioning elements, and causing relative motion between at least one of the vertically movable conditioning elements and the end effector during conditioning of the polishing pad.
Another aspect of the invention features a method for conditioning a polishing surface of a polishing pad, comprising bringing a plurality of conditioning elements that are secured by a compliant backing member to an end effector into contact with the polishing pad, and causing relative motion between the polishing pad and the conditioning elements.